The field of the invention is that of acoustic transducers which can be used especially in medical or underwater imaging, or in nondestructive testing.
Generally, an acoustic probe comprises an assembly of piezoelectric transducers connected to an electrode control device via an interconnection network.
These piezoelectric transducers transmit acoustic waves which, after reflection in a given medium, provide information relating to said medium.
Typically, within the field of medical imaging, Acoustic probes consist of numerous piezoelectric elements which can be independently excited. The method for producing such probes has been described by the applicant in several documents especially for one-dimensional probes in European Patent 0 190 948 or for two-dimensional probes in French Patent 93/02586. This method consists in cutting an assembly formed from acoustic matching plates, from a piezoelectric ceramic plate, from an electrical circuit comprising metal tracks, which is generally placed on the surface of an acoustic support known by the term xe2x80x9cbackingxe2x80x9d. The cutting thus makes it possible to define elementary transducers which can be excited independently. This is because each transducer is connected to one track of the electrical circuit (polyimide film with metallized tracks or tracks cut from a metal foil) in order to allow electrical excitation.
In order to prevent undesirable vibration modes, especially the transverse mode, the elementary transducers are subcut into several piezoelectric subelements, thus they are mechanically separated but connected to the same electrical point. The subcuts are obtained by cutting beyond the metal tracks, as illustrated in FIG. 1 which shows a sectional view of an example of a unidirectional multielement probe. According to this configuration, a backing 1 supports an electrical circuit 2 with conducting tracks pi1, elementary transducers ti1 themselves comprising subelements ti1k. Typically the width of the track Pi1 is about 100 xcexcm, which limits the number of piezoelectric subelements. Furthermore, the cut tracks are fragile and do not withstand electrical and mechanical stresses well.
The piezoelectric elements also comprise acoustic matching elements L1i1k and L2i1k with different impedances, it being possible for the element L2i1k to be metallized on the lower face in order to allow them to be earthed.
Earthing can also be carried out by inserting a thin metal film between the plate L2i1k and the ceramic or by using, in the case of the one-dimensional probes, strips L1i1k and L2i1k of smaller dimensions that those of the ceramic, thus making the earth electrode accessible on the ends of the ceramic. In the latter case, the earth is picked up by soldering or by adhesively bonding a metal film to the xe2x80x9cfreexe2x80x9d ends of the ceramic.
In order to alleviate the aforementioned drawbacks, the present invention provides an acoustic probe comprising a film of composite conducting material.
More specifically, the subject of the invention is an acoustic probe comprising elementary piezoelectric transducers and an electrical circuit comprising metal tracks, so as to connect at least one metal track to at least one elementary transducer, each elementary transducer being formed from piezoelectric subelements which are mechanically separated and connected to the same track, characterized in that it furthermore comprises a film of composite conducting material lying between the electrical circuit and the elementary transducers, the piezoelectric subelements of the same elementary transducer being mechanically separated by gaps extending right into the said film.
Conventionally, the electrical circuit of acoustic probe according to the invention is affixed to a backing of matched impedance in order to act as an acoustic support.
Such a probe has especially the following advantages:
since the gaps defining the piezoelectric subelements stopping in the film of conducting material the tracks of the electrical circuit are no longer xe2x80x9csubcutxe2x80x9d and therefore weakened;
the film of composite conducting material allows the piezoelectric elements and the electrical circuit to be electrically connected without passing through vias as described especially in French Patent 93/02586;
the film of composite conducting material, as it may have a thermal expansion intermediate between that of the piezoelectric material and that of the material forming the xe2x80x9cbackingxe2x80x9d, allows the deformations due to the thermal stresses from assembly, conventionally carried out at high temperature, to be absorbed;
the tracks of the electrical circuit no longer have to be dimensioned according to the number of piezoelectric subelements that it is desired to obtain, since the gaps stop in the film of composite conducting material.
Advantageously, the film of composite conducting material may comprise an organic material of the epoxy resin type, which may especially be filled with conducting particles made of a metal such as silver, copper or nickel.
The subject of the invention is also a process for fabricating an acoustic probe according to the invention and furthermore comprising the following steps:
the assembly of at least one layer of piezoelectric material, one film of composite conducting material and one electrical circuit comprising metal tracks;
the cutting of the layer of piezoelectric material and of the film of composite conducting material so as to define elementary piezoelectric transducers which are electrically separate;
the subcutting of the elementary transducers and of part of the film of composite material so as to define piezoelectric subelements which are mechanically separated and electrically connected.
According to one embodiment of the process of the invention, the cutting and subcutting steps may be carried out with a diamond saw and in one step.